Captcha as Graphical Passwords�A New Security Primitive Based on Hard AI Problems

Many security primitives are based on hard mathematical problems. Using hard AI problems forsecurity is emerging as an exciting new paradigm, but has been under-explored. In this paper, we present a new security primitive based on hard AI problems, namely, a novel family of graphicalpassword systems built on top of Captcha technology, which we call Captcha as graphical passwords(CaRP). CaRP is both a Captcha and a graphical password scheme. CaRP addresses a number ofsecurity problems altogether, such as online guessing attacks, relay attacks, and, if combined with dual-view technologies, shoulder-surfing attacks. Notably, a CaRP password can be found only probabilistically by automatic online guessing attacks even if the password is in the search set. CaRP also offers a novel approach to address the well-known image hotspot problem in popular graphicalpassword systems, such as PassPoints, that often leads to weak password choices. CaRP is not a panacea, but it offers reasonable security and usability and appears to fit well with some practical applications for improving online security.

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